The present invention generally relates to solar cells. More particularly, it relates to new and improved solar cells including an alkali metal doped chalcopyrite absorber layer.
A principal goal in photovoltaics is development of solar cells exhibiting good long-term stability and high efficiency coupled with low manufacturing costs. Only by achieving these combined characteristics can this form of generating energy be competitive with other, conventional methods.
Solar modules of single-crystal silicon are currently considered the standard with respect to efficiency and long-term stability.
Promising beginnings for manufacturing solar cells at beneficial cost with long-term stability are currently suspected for solar cells including polycrystalline I-III-VI.sub.2 chalcopyrite semiconductors. In particular, CuInSe.sub.2 (CIS) and related alloys from the system CuIn(Ga)Se.sub.2 (S.sub.2)(=CIGS) are materials that comprise a high absorption coefficient, a direct band transition, and a band gap matched to the solar spectrum. They can be manufactured by cost-beneficial thin-film deposition methods and have already achieved efficiencies of above 17% in laboratory tests.
A standard cell structure is composed of a stacked laminate including a glass substrate, a molybdenum back electrode, a 1-5 .mu.m thick absorber layer of polycrystalline chalcopyrite semiconductor, a thin cadmium sulfide window layer, and a transparent front electrode.
It has been shown that record efficiencies can only be obtained with solar cells that are manufactured on alkali-containing glass substrates, for example, calcium soda-lime glass. A negative influence on the grain growth of the chalcopyrite absorber layer is found with other substrate materials. Given manufacturing conditions that are otherwise identical, fine-grain crystalline layers having average grain diameters of approximately 200 nm are obtained with other substrate materials. Additionally, especially fine-grain crystals having a maximum size of approximately 50 nm and different coverage area form at the surface of the chalcopyrite layer. These cause a reduction of the cell current, of the maximally obtainable no-load voltage, of the short-circuit current densities, and of the fill factor.
Contaminants deriving from the substrate are suspected as the cause for the dependency of the solar cell properties. Since the substrate is heated close to the softening point in the layer formation of the absorber layer, it is particularly alkali metal and alkaline earth metal ions that can diffuse out of the substrate into the semiconductor. Standard substrates of float glass comprise alkali metal and alkaline earth metal concentrations of about, for example, 13-15% sodium, 5-7% calcium, 4-5% magnesium, and 0.5-1% potassium.
The problem arises in the manufacture of solar cells with chalcopyrite absorber layer that the positive influence of calcium soda-lime glass on the cell properties is additionally dependent on the pre-treatment of the substrate material, for example on the cleaning of the surface, on the storage conditions with respect to moisture, duration, and temperature of both the uncoated substrates as well as the substrates coated with a molybdenum back electrode. The deposition conditions of the molybdenum back electrode also influence the solar cell properties. It is suspected that different deposition conditions primarily influence the diffusion behavior with respect to alkali and earth-alkali ions. Moreover, the amount of heat employed during manufacture of the chalcopyrite absorber layer is a further factor for the solar cell properties obtained.
It should be cited as a further disadvantage that the chalcopyrite layers often have poor adhesion on the molybdenum back electrode.
A 100% process control that is required for industrial scale manufacture of high-grade solar cells with chalcopyrite absorber layers is practically possible only with extremely great outlay.